Abstract
BACKGROUND: Cognitive impairment, a core feature of schizophrenia, is often evident before the onset of illness. The current study aimed to quantify IQ decline following the onset of illness by conducting a meta-analysis of longitudinal studies that evaluated cognitive functioning both before and after the first psychotic episode. Consistency in measurement tools - i.e. whether the same measurement tool was used at both assessments - was considered a potential moderating variable. METHOD: Eleven studies were included in the meta-analysis - seven using the same measurement tool at both time-points and four using different tools. In addition, meta-regression explored whether the magnitude of IQ decline was associated with age at baseline. RESULTS: The meta-analysis effect size was -0.343 (95 % CI: -0.503 to -0.184), equivalent to a decrease of 5 IQ points. Use of the same (SMD -0.321, 95 % CI: -0.501 to -0.142) vs different (SMD -0.427, 95 % CI: -0.777 to -0.077) measurement tools was not a moderator of IQ change (p = 0.279). The meta-regression results were not significant (p = 0.544). CONCLUSION: The current meta-analysis indicates a slight cognitive decline from the premorbid stage to post-onset. The use of different measurement tools yielded a slightly larger effect size and greater heterogeneity, suggesting that employing the same assessment tool could lead to more accurate results. Future longitudinal studies should focus on determining the timeline of cognitive decline.